I had been having problems getting a hot shower for a while, but when the hot water around the rest of the home started failing as well I replaced the heat-exchange plate in my combi-boiler.

When I did this the shower was instantly hot again (even hotter than before because I'd been adjusting the thermostat in the mixer to try to solve the original issue) because the heat-exchanger had got all scaled up.

But it wasn't long before the shower started getting unreliable. You can turn it on, it gets fairly hot, then gets cool for a few minutes before maybe returning to warm/hot and then it may stay hot for the rest of the time its on. And then sometimes it might not get warm/hot at all.

When it does get warm, the temperature isn't really adjustable either (it used to be). I've tried adjusting the thermostat but it hasn't made any difference. The thermostat is like a small screw with some rubber over one end. You screw it anti-clockwise to make it hotter etc. but I don't really understand how it performs its function of regulating the temperature. Can this type of thermostat fail, or is it an issue in the main part of the faucet?

Thanks very much for anyone who reads, and especially to any input given.

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